What the rail will do: Cost $150 -300. It then accepts a light or other accessory.
What the rail won't do: improve accuracy beyond the undisturbed MOA the barrel already had. Milspec is 2MOA, it cuts about a 1/2MOA deviation if you sling up tightly in long range shooting: hand on the mag well, about zip.
That's one expensive flashlight mount. The Army does it as an institutional compromise to clamp all that "SOCOM" kit on 50 different user inventories. The Marines kept it optional on the M16A4 and still use handguards.
The key phrase is "rail height," meaning the rail will be the same as the one on the upper. Most are, the few that aren't are obvious. What many won't do well is bridge the gap from upper to rail with exactly even spacing that still accepts a mount, which is moot, as noone recommends doing it anyway. If the optic is that long, they recommend a cantilever base.
Boils down to, why quad rail it when the optics and mount makers don't recommend bridging, and offer rail strips to mount lights as you need it? It comes down to being a free float tube, and that will only protect the barrel against sling or bipod tensioning. Shooting at those ranges implies something more than the standard 2MOA chrome barrel - and would be the first item to spend money on.
When you really get into understanding why the Army does some things, it's not what we think at all.
